Use go modules

[ghost]

Dep has been deprecated as of September golang/go#38158 (comment) 2020. This PR removes dep and switches the project to use Go modules.

[ooq]

Thanks for cleaning this up! Can you fix the travis test, I think it's complaining because some vendor script is missing after the update: https://travis-ci.org/github/grafeas/kritis/builds/747482496

[ooq]

Looks like it's still failing :( @cjones-shopify

[ghost]

The Travis-CI pipeline takes quite a long time to run, I'm sorry it's taken so long for me to fix this PR! :<

[ooq]

No worries, @cjones-shopify! I've recently noticed that Travis might not pass back the build status back -- not sure why. I'll check when this rerun finishes.

[ooq]

Looks like it's being queued forever :(

[ooq]

Hi just a quick update that the travis issue seems common due to their migration phase from travis-ci.org to travis-ci.com. I just fixed it by completing the migration.
The test looks good now (the Travis CI - Pull Request check) and you can ignore the continuous-integration/travis-ci/pr check.
Sorry for the confusion!

[ghost]

Thank you so much @ooq!